OOP Features

Classes and objects and Methods and properties

A class is a user defined data type like a structure or a union. A class consists of data variables and functions. These variables and functions are called members of the class.  The variables are called data members and functions are called member functions. The member functions are also called methods. The data members are called properties of the class. An object is the instance of the class. An object is like a compound variable of the user defined type. It links both code and data. Within the object, members of the class can be public or private to the object. The declaration of a class is syntactically same as structure. The class is declared using keyword class. The general form of the declaration of the class is:-

class class_name

{

            access_specifier:

                        data functions

            access_specifier:

                        data functions

} object_list;

The object_list is optional. The object_list is used to declare objects of the class. The class_name is the name of the class.  The access_specifier can either public, private or protected. The members of the class by default are private to the class. If the access_specifier is private then members of the class are not accessible outside the class. If the access_specifier is public then members of the class can be accessed from outside the class. The protected access_specifier is needed at the time of inheritance. The members can be accessed using an object’s name, a dot operator and name of the member. Here is a program which shows how classes and objects are created.

#include<iostream>

using namespace std;

class cube

{

            public:

                        double side;

                        double volume()

                        {

                                    return(side*side*side);

                        }

};

int main()

{

            double volume1=0;

            cube c1,c2;

            cout << "Enter the lenght of the cube" << endl;

            cin >> c1.side;

            cout << "The volume of the cube is : " << c1.volume() << endl;

            c2.side=c1.side +2;

            cout << "The volume of the second cube is : " << c2.volume() << endl;

            return(0);

}

The result of the program is:-

The program consists of a class cube which has data member side of type double and member function which calculates the volume of the cube. The statement

            class cube

declares a class cube. The statements

            public:

                        double side;

                        double volume()

                        {

                                    return(side*side*side);

                        }

declare that access_specifier is public for data member side and member function volume. These members can be accessed from the other parts of the program. The statement

            cube c1,c2;

declares two objects c1 and c2 of type cube. The statement

            cin >> c1.side;

access the data member of the cube. The member is accessed by specifying the name of the object as c1 then dot operator and then name of the variable side. The length entered by the user is stored in c1.side. In the statement

            cout << "The volume of the cube is : " << c1.volume() << endl;

c1.volume() calls the member function volume which returns the volume of the cube of side whose length is entered by the user. The statement

            c2.side=c1.side +2;

equates the side of object c2 to side of object c1 increased by 2.  The objects c2 and c1 are different. The statement

            cout << "The volume of the second cube is : " << c2.volume() << endl;

displays the volume of second object c2.

Constructor and Destructor

Constructors are used in order to initialize the objects. A constructor is a special kind of a function which is the member of the class. The name of the constructor is same as name of the class. A constructor is automatically called when object is created. A constructor does not have a return type.

A default constructor is a constructor with no parameters. If no constructor is defined by the user then compiler supplies the default constructor. Once the constructor is defined by the user then compiler does not supply default constructor and then user is responsible for defining default constructor. 

A destructor is the complement of the constructor. It is used to destroy the objects. The objects are destroyed in order to deallocate the memory occupied by them. The name of the destructor is same as the name of the constructor as is preceded by a tilt operator ‘~’. A destructor for objects is executed in the reverse order of the constructor functions.

Here is a program which shows how constructors and destructors are used.

#include<iostream>

using namespace std;

class cube

{

            public:

                        double side;

                        double volume()

                        {

                                    return(side*side*side);

                        }

                        cube(double side1)

                        {

                                    cout << "A constructor is called" << endl;

                                    side=side1;

                        }

                        cube()

                        {

                                    cout << "A default constructor is called " << endl;

                        }

                        ~cube()

                        {

                                    cout << "Destructing " << side << endl;

                        }

};

int main()

{

            cube c1(2.34);

            cube c2;

            cout << "The side of the cube is: " << c1.side << endl;

            cout << "The volume of the first cube is : " << c1.volume() << endl;

            cout << "Enter the length of the second cube : " ;

            cin >> c2.side;

            cout << "The volume of second cube is : " << c2.volume() << endl;

            return(0);

}

The result of the program is:-

The statement

            cube(double side1)

            {         

                                    cout << "A constructor is called" << endl;

                                    side=side1;

            }

declares the constructor of the class cube. The name of the constructor is same as the name of the class. There is no return type in the constructor. It will initialize the value of data member side. The statement

cube()

                        {

                                    cout << "A default constructor is called " << endl;

                        }

declares a default constructor. The statement

                         ~cube()

                        {

                                    cout << "Destructing " << side << endl;

                        }

declares a destructor to deallocate the objects. The statement

            cube c1(2.34);

creates an object c1 of type cube. A constructor is automatically called and initializes the data member side with value 2.34. The statement

                        cube c2;

creates an object of type c2. When object c2 is created a default constructor is called and the message will be printed. The statements

            cout << "The side of the cube is: " << c1.side << endl;

            cout << "The volume of the first cube is : " << c1.volume() << endl;

displays the side and volume of the cube where side has value 2.34. The statement

            cin >> c2.side;

will set the value of the side of the object c2 as entered by the user. At the end of the program objects are deallocated in the reverse order in which constructors are called. First object c2 is deallocated whose side is 2.5 and then object c1 is deallocated whose side is 2.34.

Advantage of the classes:-

It provides protection to the data. The members of the class are by default private to the class while the members of the structure are public. OOP features allow programmer to easily handle complex problems and multi file projects. They help in modeling real world objects such as bank accounts and their related transactions.